Receiver manual and automatic gain control circuits



May 5, 1936. H A. ROBINSON RECEIVER MANUAL AND AUTOMATIC GAIN CONTROLCIRCUITS Filed No v. 5, 1954 q -w T whllnmwl l I! W x (v \m Kg mw n UHNQN mw m g 07 "Q Iv. W w [M g M 3 m 3 3 W v *QNNQQ gm Patented May 5,1936 UNITED STATES PATENT OFFICE RECEIVER MANUAL AND AUTOMATIC GAINCONTROL CIRCUITS of Delaware Application November 3, 1934, Serial No.751,317

7 Claims.

My present invention relates to circuit arrangements for selectingautomatic or manual gain control in a radio receiver, and moreparticularly to circuit arrangements of this type which are adapted forairplane radio receivers.

One of the primary objects of the presentinvention is to provide asimple and efficient arrangement for permitting the selection ofautomatic or manual volume control for radio receiving equipment whereina simple switching arrangement is utilized, and the same physical volumecontrol is employed in both positions of the switching arrangement. I

Another important object of this invention is to provide a circuitarrangement for gain control of radio receivers wherein the simplicityof the switching, and the requirement of a single volume controlpotentiometer for both the manual and automatic functions are importantadditions.

- Another object of the invention is to provide a volume control devicefor a radio receiver which is capable of being manually adjusted, andwhich functions as a sensitivity control by varying the bias on thecontrol grids of the radio frequency and/or intermediate frequencyamplifier tubes, and wherein the same volume control functions to adjustthe audio output level when a simple selector switch is utilized toplace the receiver under the control of an automatic volume controlcircuit.

Still another object of the invention is to provide a simplified circuitarrangement for selection of automatic or manual volume control, thearrangement being particularly applicable on radio receiving equipmentdesigned for reception of telephone or other modulated signals involvingan uninterrupted average carrier amplitude where AVC is desirable, withquick change to manual control for reception of C. W., or modulatedsignals, in which the carrier is interrupted periodically.

Still other objects of the invention are to improve generally thesimplicity and efliciently of circuit arrangements for selectingautomatic or manual volume control in radio receivers, and toparticularly provide such circuit arrangements which are not onlyefficient. in operation, but economically constructed and assembled inradio receivers.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically a circuit organization whereby my invention may becarried into effect.

Referring now to the accompanying drawing, which represents a typicalcommunication re- 5 ceiver particularly adapted for aircraft use, thereceiving system generally embodies an R. F. amplifier, a firstdetector, a local oscillator, an I. F. amplifier, a combined seconddetector and AVC tube, and. an A. F. amplifier. The circuits associatedwith each of these successive stages are conventional in nature, and forthis reason these circuits are shown in such a manner that those skilledin the art will readily understand their functions, but they will not bedescribed in any detail. The superheterodyne receiver shown is to beunderstood as being applicable not only to aircraft receivers, but tonumerous commercial receivers which are usually employed for C. W. andmodulated reception.

In general, the purpose of the present invention is to control the gainof the receiver automatically, or manually, and to provide a simpledevice for switching the receiver from manual to automatic gain control,and, furthermore, to provide two types of manual gain control; onefunctioning when the AVG network is operated, and the other functioningin the absence of AVG operation. These two types of manual gain controlutilize, in the interests of simplicity, a common manually adjustablepotentiometer. The switching ar rangement and the manually adjustablepotentiometer are both disposed in a control box which is connected. tothe receiver proper by means of a shielded cable; the control box,additionally, being connected to a power supply unit (not shown) througha shielded power cable.

The essential circuit features of the superheterodyne receiver will nowbe described, it being noted again that circuit details are omittedbecause they do not constitute a part of the present invention. Thereceiving system is adapted to operate in a frequency range of 2000 to6600 kilocycles, and utilizes exponential pentode tubes of the 39 typein the R. F. and I. F. amplifier stages, as well as in the firstdetector stage. The multiple function tube used in the second detectornetwork is of the type, while the A. F. amplifier tube is a powerpentode of the 89 type. The second detector tube 85 includes a diodeanode 86 which is connected by lead 81 to the signal grid circuit of R.F. amplifier tube 39, and to the signal grid circuits of first detectortube 39 through resistor R2 and lead 88.

The lead 88 is also connected to the signal grid 55 circuit of the I. F.amplifier 39". When the oathode of tube 85 becomes negative with respectto the anode 85, then the signal grids of the Com trolled tubes 39, 39',and 39" are simultaneously rendered increasingly negative with respectto the cathodes of these tubes. This results, of course, in a reductionin the gain of the controlled tubes, and a consequent decrease in thevolume of the receiver. The automatic volume control arrangement shownis incorporated in the receiver to reduce the effects of fading which isprevalent in the frequency range in which thereceiving system shownoperates. The automatic volume control arrangement additionallyminimizes the frequent adjustment of the volume control when approachingor leaving the airport transmitters.

The tuning condensers of the radio frequency amplifier, first detectorand local oscillator have their rotors arranged for mechanicaluni-control, as shownin dotted lines, and. the tuning control may be ofthe remote control type, with the control unit mounted on the controlbox. Additionally, the wave changing switches designatedby the numeral10 may be mechanically uni-controlled, and adjusted from the controlbox. The control box is compact, and can be mounted in any small, thougheasily accessible, place in an airplane. For remote control tuningflexible tuning shafts are supplied,'and are easily fitted to have thetuner-adjusted from the control box. The flexible power cable andreceiver cable :which run to the control box are fitted with removableplugs and receptacles These plugs and receptacles are conventionallyrepresented in the drawing, and it is believed that theirrepresentations are self-explanatory.

The male plug on the control box receives a female plug on the powercable connecting to the power supply unit, while the receiver cableterminates in a male plug fitting thefemale receptacle on the controlbox. The flexible tuning shafts are'supplied with aslotted end fittingwhich engages with a pin on the tuning control shaft (or the band changeswitch) in the control box. Short slotted'shafts are furnished forinterconnecting the tuning and band change controls on the control box(not shown to preserve simplicity of drawing) with their respectiveshafts in the receiver proper. These various control constructions arewell known to those skilled in the art,

' and it is believed that the aforegoing brief description will besuflicient for the purpose of this application. t

Thepower supplyunit maybe of any well known type, and, for example, avibrator-rectifier unit may be employed. In such a construction a fullwave vibrator breaks the primary current, and the'correspondinglyinduced secondary voltage is rectified by an 84 type full waverectifier. A suitable filter removes the ripple and radio frequencyinterference. If a dynamotor unit is employed as the power supply unit,a suitable filter is utilized in connection with the generator of theunit for reducing the commutator ripple and radio frequencyinterference. Both of these types-of'power unit can be supplied foroperation from a 6 volt battery, as well as the usual 12 voltairplanebattery. In general, the power supply unit'should be located fairlydistant from the receiver antenna lead, and as near the airplane batteryas possible. The antenna A should be located as far as possible from themotors and other source of electrical interference. The antenna is astandard airplane vertical pole antenna, andthe receiving system employsa good short, direct, low resistance ground to the frame of theairplane. I

The cathode of the second detector tube 85 is connected to the Bterminal of the control box and power supply through a path whichincludes resistor R15 and lead 5'. The grid of tube 85 is connected tothe anode side of resistor R11, and for this reason becomes increasinglynegative in potential with respect to the cathode. of the same tube asthe received signal amplitude increases.

As the signal grid of tube 85. becomes more and more negative, the spacecurrent flow through resistor R15 decreases, with the result that thecathode of tube 85 becomes negative with respect to anode 86. Thus, whenanode 86 becomes positive with respect to the cathode of tube 85 a flowof electronic current occurs through the AVG network including resistorsR2 and R3 and the signal grids of the controlled tubes becomeincreasingly negative with respect to their cathodes. In other words, upto a predetermined intensity of received signal amplitude the anode 86is essentially negative with respect to the cathode of tube 85. Whenthis predetermined signal amplitude level is exceeded the voltage ofanode 86 becomes positive with respect to the cathode of tube 85.

Received signals, collected by antenna T, are impressed upon the R. F.amplifier tube 39. This tube, of course, has a normal signal grid biaswhich is produced by the usual cathode circuit resistor. The amplifiedsignal energy is then impressed upon the signal input circuit of thefirst detector tube 39', and local oscillations are impressed upon thecathode coil L1 disposed in L.

the cathode circuit of the first detector tube. The local oscillator isa triode of the 31 type, and its circuits are too Well known to requireany detailed explanation. The coil in the grid circuit of the oscillatoris coupled tothe cathode coil L1.

The beat frequency energy, at the operating intermediate frequency, istransmitted to the amplifier 39 through the tuned coupling network M1,and the amplified intermediate frequency energy is impressed upon thetuned diode detector circuit of tube 85 through the tuned couplingnetwork M2. There is impressed upon the signal grid of tube 85 bothdirect current and audio frequency componentsof the detected signalenergy, and the audio component is transmitted through coupling networkM3 to the audio amplifier 89. The audio frequency component isamplified, and then impressed through audio frequency transformer T1upon anydesired type of reproducer.

The switch S in the control box is the volume control selection switch,and is of the doublepole, double-throw type. One arm of this switch isgrounded, while the other arm is connected to a tap P which isadjustable along resistorRmz. One pair of selection switch contacts arecontacted by the arms of the selection switch for the operation of theAVG network, and this position of the selection switch, as Well as thecontacts, is represented by the reference character A. The other pair ofcontacts are designated by the reference character M. One of thecontacts A is connected through lead I00 and resistor R3 to the AVGleads 88 and 81. The other contact of this pair of contacts A isconnected by the condenser C100 to the telephone jacks. One of the pairof contacts M is connected to the contact adjacent condenser C100, whilethe remaining contact of this pair is connected to AVC lead 88.

The resistor R102 is connected between the telephone jacks and the platecircuit of the audio amplifier 89. One end of resistor R102 is grounded,and a resistor R14 connects the telephone jacks and the secondarywinding of transformer T1 to the -B side of resistor R15.

When the switch S is thrown to the M position, the gain of the receivercan be manually adjusted by moving tap P. This adjustment varies thenegative bias voltage of the radio frequency amplifier, first detectorand intermediate frequency amplifier. When the switch S is thrown intothe A position, or the AVG position, adjustment of the tap P varies theaudio output level of the receiver. In other words, adjustment of tap Pin this A position of the selector switch does not affect the negativegrid bias voltages of the tubes 39, 39' and 39".

Considering the circuit details of the two positions of the volumecontrol selector switch, it is first pointed out that in the M positionof selector switch S the audio output circuit iscom pleted through thecapacitor C100 to ground. The arm of the tap P is, in this position ofthe selector switch, connected to the grid return leads of the tubes 39,39 and 39" through the radio frequency filter resistors R1, R2 and R0.The negative control voltage is obtained at the volume control throughthe cable lead 1 and the audio filter resistor R14 which is connected tothe negative (ungrounded) terminal of the B supply (180 volts). In thisposition of the volume selector switch the end of resistor R3 isdisconnected from ground, thus preventing the functioning of the AVGnetwork.

When the switch S is thrown into the A position, the AVG position, theswitch grounds resistor R3 and completes the circuit across which theAVG anode 86 of tube 85 can develop negative potential depending uponthe signal amplitude. This position of the selector switch providesautomatic gain control of the radio frequency amplifier, the firstdetector and the intermediate frequency amplifier. The tap P is thenconnected through the capacitor C100 to the output telephone jacks, andadjustment of the tap along resistor R102 varies the audio volume level.In this position of the selector switch S movement of the tap P alongresistor R102 does not result in any variation of the negative grid biasof tubes 39, 39' and 39". The capacitors C100 and C25 are capacitorswhich readily transmit the audio frequencies, and act to keep the Bvoltage from shorting through the output transformer secondary 296.Filter resistor R14 is required to isolate the audio output circuit fromthe relatively low impedance by-pass to ground on the -B supply.

It is believed that the operation of the circuit arrangement includingthe present invention will be clear from the aforegoing description andthe circuit diagram of the drawing. The low impedance head telephonesare inserted in the jacks, and the receiver energized by means of theOn-Off switch in the control box. The dial lamp in the control box showsby its brightness whether the filaments of the tubes are up to theiroperating temperature. Assuming that the band change switch has beenthrown to the desired frequency band, the receiver is tuned rathercarefully because of the extreme degree of selectivity which is providedin this receiver. When the receiver is being tuned the selector switch Sis thrown to the AVG position, and the tap P can then be adjusted tovary the audio level, and a suitable volume level may be easilyselected. This is used particularly for the receptionoftelephone-orother modulated signals involving an uninterrupted voltage carrieramplitude. When it is desired to receive C. W., or.

modulated signals in which the carrier is interrupted periodically, itis only necessary to throw the switch S to the M positionand control thegain, or sensitivity, of the receiver by'varying the tap P.

' While I have indicated and described a system amplifier and an'outputdevice, means for automatically controlling the gain of said radiofrequency amplifier, means including a manually adjustable resistancefor controlling audio transmission through said receiver, meansincluding a manually adjustable resistance for controlling thesensitivity of said radio frequency amplifier, switch means adjustableto one position to render said automatic gain control and said audiotransmission control means operative while rendering said sensitivitycontrol means inoperative and adjustable to a second position to rendersaid automatic gain control and said audio transmission control meansinoperative while rendering said sensitivity control means operative,both said manually adjustable resistances consisting of a commonvariable resistor.

2. In a radio receiver, the combination with a radio frequencyamplifier, a detector, an audio amplifier and an output device, meansfor automatically controlling the gain of said radio frequencyamplifier, means including a manually adjustable resistance forcontrolling audio transmission through said receiver, means including amanually adjustable resistance for controlling the sensitivity of saidradio frequency amplifier, switch means adjustable to one position torender said automatic gain control and said audio transmission controlmeans operative while rendering said sensitivity control meansinoperative and adjustable to a second position to render said automaticgain control and said audio transmission control means inoperative whilerendering said sensitivity control means operative, the manuallyadjustable resistance of said audio control means being the adjustableresistance included in the radio frequency sensitivity control means.

3. In a radio receiver, the combination with a radio frequencyamplifier, a detector, an audio amplifier and an output device,adjustable means for controlling the sensitivity of said radioamplifier, adjustable means for controlling the ratio of the outputlevel at said output device to the input level at said detector, andswitch means adjustable to alternative positions to render either ofsaid adjustable means operative and the other inoperative, and both ofsaid control means including a common adjustable resistor.

4. In a radio receiver, the combination with a radio frequencyamplifier, a detector, an audio amplifier and an output device, anadjustable means for controlling the sensitivity of said radioamplifier, adjustable means for controlling the ratio of the outputlevel at said output device to the input level at said detector, bothsaid adjustable means comprising a common variable resistor, and switchmeansadjustable to alternative positions to render either of saidadjustable :means operative and the other inoperative, a control boxremote from the two amplifiers and detector, and said aifiustable meansand switch means being disposed in said control box.

5. In combination with a radio frequency amplifier, a detector, and anaudio frequency am-- plifier, manually operable means for controllingthe volume of the output of the audio amplifier, automatic means forcontrolling the volume of the audio amplifier irrespective of theamplitude of incoming signals, manually operable means for controllingthe sensitivity of the radio frequency amplifier, both of said manuallyoperable means having a variable impedance in common, and manuallyoperable means for selecting the type of volume control desirable.

6. In combination, in a radio receiver of the type including a radiofrequency amplifier, a first detector, an intermediate frequencyamplifier, a second detector, an audio frequency amplifier, and anoutput device, manual sensitivity control means for varying the signalgrid bias of the radio frequency and intermediate frequency amplifiersand the first detector, manual volume control means for varying theaudio frequency output level in the audio frequency amplifier, both saidmanual control means including a common variable resistor, and switchmeans for selecting the type of manual volume control desirable.

7. In combination, in a radio receiver of the type including a radiofrequency amplifier, a first detector, an intermediate frequencyamplifier, a second detector, an audio frequency amplifier, and anoutput device, manual sensitivity control means for varying the signalgrid bias of the radio frequency and intermediate frequency amplifiersand the first detector, manual volume control means for varying theaudio frequency output level of the audio frequency amplifier, both saidmanual control means including a common variable resistor, switch meansfor selecting the type of manual volume control desirable, and anautomatic gain control network, operatively associated with said manualaudio volume control, adapted to be rendered operative when the manualaudio volume control is rendered operative.

HARRIS A. ROBINSON.

